1. Field of the Invention
This invention pertains to sharpening equipment, and more particularly to apparatus that sharpens a wide variety of cutting blades.
2. Description of the Prior Art
Rotary power mowers of various types are well known and are in widespread use. They vary greatly in size, complexity, and operating characteristics. However, all rotary power mowers utilize the common component of a rotary cutting blade. The blades are relatively long, narrow, and thin. A cutting edge is sharpened into each end of the blade. As the blade rotates under power from the mowing machine, the cutting edges shear vegetation.
The cutting edges of some rotary blades lie along substantially straight lines for their full lengths. A typical example of such a blade is shown at reference numeral 3 in FIGS. 1-3. The blade 3 has a longitudinal centerline 4 and a flat bottom surface 6. There are a pair of cutting edges 8, both of which lie along straight lines 10 that are substantially parallel to the longitudinal centerline 4. The cutting edges 8 and the straight lines 10 coincide.
Each cutting edge 8 is defined by the junction of the blade bottom surface 6 and a cutting surface 12. The bottom surface 6 and cutting surface 12 subtend a cutting angle L. Each cutting edge 8 intersects an end 34 of the blade 3 at a tip 38. The tips 38 are especially important, because they are the portions of the blade 3 that first come into contact with the vegetation being cut by a power mower. The blade has back edges 2 opposite the cutting edges 8. There are an infinite number of imaginary lines 14 along the blade bottom surface 6 between the cutting edge 8 and the back edge 2. Flat blades such as the blade 3 are used to shear vegetation and expel it tangentially away from a mowing machine.
Other cutting blades have cutting edges that are not straight but instead lie along curved lines. The curved cutting edges are particularly useful for mulching the vegetation in addition to shearing it and expelling it. FIGS. 4-7 show a typical mulching blade 16 having five sections: a relatively long center section 26, a pair of angled sections 28 that slope downwardly and away from the ends of the center section 26, and a pair of end sections 30 on the ends of the angled sections 28. The end sections 30 lie in a plane that is substantially parallel to the plane of the center section 26. The center section has a bottom surface 22, the angled sections have respective bottom surfaces 48, and the end sections 30 have respective bottom surfaces 49.
The mulching blade 16 further has curved cutting edges 18. Each cutting edge 18 lies along a respective curved line 20. The cutting edges 18 and lines 20 are defined by the junctions of the bottom surfaces 22, 48, 49 of the blade sections with associated cutting surfaces 24. The cutting edges 18 intersect the ends 40 of the mulching blade at tips 42. There are an infinite number of imaginary lines 32 on the mulching blade bottom surfaces between the cutting edges 18 and the back edges of the blade opposite the cutting edges.
A routine task associated with rotary power mowers is sharpening the blades. For that purpose, a variety of mechanized equipment has been developed. For example, a division of Wall Enterprises, Inc. of New Whiteland, Ind., markets rotary blade grinding machines. Magna-Matic Corporation of Waldo, Wis., is a manufacturer of blade grinding and balancing machines.
Prior blade sharpening machines have not been entirely satisfactory. As one reason, they generally have been limited to sharpening blades with straight cutting edges, i.e., flat blades such as blade 3 depicted in FIGS. 1-3. That is because the prior machines had flat work surfaces of substantial area and lying in a single plane. The blades were supported on the work surfaces and manually fed lengthwise, i.e., parallel to the longitudinal centerline 4 of the blade 3, while in contact with a grinding wheel. The flat area of the work surface was satisfactory for straight edge blades, because no angular movements of the blade in space were required to grind the cutting edge.
On the other hand, the flat area work surfaces of prior blade sharpeners presented considerable difficulty with mulching blades, such as mulching blade 16 of FIGS. 4-7. That is because the mulching blade curved edges 18 required that the blades be fed crosswise in addition to lengthwise in order to properly grind the curved cutting edges. Simultaneous crosswise and lengthwise feeding of the blades was difficult to do, especially at the transitions between the various sections, such as the transitions between the center section 22 and the angled sections 28, and the transitions between the angled sections and the end sections 30. The prior sharpeners used relatively wide grinding wheels, such as one inch, which exacerbated the problem. Moreover, with a flat work surface of substantial area, the curved cutting edges of the different sections contacted the grinding wheel at different angular locations on the grinding wheel periphery. As a result, the cutting angles of the cutting edges at the different blade sections varied considerably. A uniform cutting angle was possible only by carefully imparting angular motions to the blade while lengthwise and crosswise feeding it.
An associated problem with the large flat work surface of a prior machine is that the work surface is cut out to make room for the grinding wheel. The work surface is therefore rather limber and prone to vibrate and create excessive noise during operation. A related problem with the cutout in the work surface is that the blade cutting edge is not supported directly under the grinding wheel. It is very difficult to properly grind the tips of a blade using such a machine, because the cutting force of the grinding wheel is not resisted by any force except that produced by the operator. Even if the operator does not yield to the cutting force, the blade itself can bend because it is only cantilever supported by the work surface. Since the tips of the cutting edges are the most important part of the blade, any error in grinding the tips is quickly reflected in substandard mowing machine performance.
U.S. Pat. No. 5,329,731 shows an attachment for a grinding machine that facilitates sharpening mulching blades. The attachment has two flat work surfaces, each of substantial area, along which a blade is fed. However, the flat large area work surfaces limit the attachment""s usefulness when sharpening mulching blades having certain contours. In fact, some style mulching blades can not be sharpened at all when using the attachment of the U.S Pat. No. 5,329,731 patent. For some other mulching blades, even using the attachment of the U.S. Pat. No. 5,329,731 patent produces varying cutting angles on the blade cutting edges along different sections of the blade.
Yet another deficiency of prior sharpening machines concerns the grinding wheels and their mountings. In prior machines, the grinding wheels were mounted directly to the armature shafts of electric motors. Accordingly, the grinding speed was limited to the motor speed. However, many grinding wheels are capable of cutting at higher speeds than conventional motor speeds. Hence, the full cutting capabilities of the grinding wheels were not used. A related problem concerns the composition of the grinding wheels used with prior blade sharpeners. To compensate for the lower production obtained by slower than usable grinding wheel speeds, prior sharpeners used harder than necessary grinding wheels. Such grinding wheels tended to burn rather than cut the metal from the blades. Further, the excessively hard grinding wheels easily loaded up with steel particles from the blades. Consequently, the grinding wheels had to be dressed periodically, which resulted in unproductive time.
Perhaps the biggest problem with prior blade grinding machinery is the lack of ability to grind blades of any length and configuration. For example, the grinding machine shown in U.S. Pat. No. 5,329,731 has upstanding posts that restrict lengthwise and even crosswise feeding of a blade past the grinding wheel. Consequently, the size and type of blades that are sharpenable are limited.
Thus, a need exists for improvements in blade grinding machines.
In accordance with the present invention, a blade sharpener is provided that is capable of properly and efficiently grinding both curved and straight edge blades without restriction. This is accomplished by apparatus that includes a work surface that supports a blade only along a single straight line, but that support line is directly opposite the surface of the blade being sharpened.
According to one aspect of the invention, the support line is a longitudinal line along a cylindrical surface on the top of a pedestal. The pedestal is mounted to a base. A pivot plate is connected to the base for pivoting about a horizontal axis. An electric motor and a grinding wheel are secured to the pivot plate. The grinding wheel is relatively narrow, and it is driven by a belt from the motor. The grinding wheel defines a plane that is parallel to and that contains the support line of the work surface. An adjustment mechanism pivots the pivot plate to adjust the position of the grinding wheel relative to the work surface.
To use the blade sharpener, the pivot plate is adjusted to bring the grinding wheel periphery close to the work surface. A blade to be sharpened is oriented such that its longitudinal centerline is generally horizontal and perpendicular to the work surface. The blade cutting edge is also usually perpendicular to the work surface. The end of the blade is placed on the work surface. The blade is fed crosswise into contact with the grinding wheel periphery such that the cutting edge at the blade tip is sharpened. The support by the work surface directly under or very close to the cutting edge at the blade tip minimizes any problems associated with sharpening the tip. The blade is then fed lengthwise along the grinding wheel periphery, all the while being supported only by the single support line of the work surface. When an angled section of the blade, and the corresponding curve in the cutting edge, reaches the work surface, the single support line enables the blade to rise and fall relative to the grinding wheel while keeping the blade cutting edge always in contact with the grinding wheel and always supported directly under or close to the blade surface being cut by the grinding wheel. In that manner, blades of a wide variety of contours and cutting edges can be efficiently sharpened.
Further in accordance with the present invention, blades having cutting edges of unlimited length are sharpenable. The blade sharpener has no posts or other members that restrict the length of the blade cutting edge. Similarly, there is no crosswise restriction in the access of the blade to the grinding wheel.
It is a further feature of the invention that it takes full advantage of modern grinding wheel technology. As one example, the production rates available from high grinding wheel speeds now available are utilized by means of an updrive of the belt drive between the motor and the grinding wheel. Consequently, rather than being limited to the speed of the motor, the grinding wheel is rotated approximately 20 percent faster than in prior blade sharpening machines. At the same time, the grinding wheel has a hardness only slightly greater than that of steel blades. The combination of the relatively soft grinding wheel and higher grinding wheel speed results in rapid and clean cutting of the blade during the sharpening process. The tendency of burning metal, rather than cutting it, from the blade that results from using the prior hard grinding wheels at slower speeds, is eliminated. In addition to having higher production, the blade sharpener of the invention eliminates the need for a grinding wheel dresser.
According to another aspect of the invention, a flat work surface of substantial area is interchangeable with the single support line work surface. For that purpose, a mobile work table has a top plate from which depend a pair of parallel lugs. The lugs are spaced apart a distance slightly greater than the width of the pedestal that includes the single line work surface. A clamp is joined to the lugs. By placing the mobile work table over the pedestal and actuating the clamp, the large area flat work surface is retained on the blade sharpener. The mobile work table finds use when sharpening flat workpieces such as straight rotary mower blades.
The method and apparatus of the invention, using a single line work support surface, thus sharpens mulching blades in an efficient manner. The single support line enables blades of practically any size and contour to be sharpened, even though the blades are supported directly opposite the blade surface being sharpened by the grinding wheel.
Other advantages, benefits, and features of the present invention will become apparent to those skilled in the art upon reading the detailed description of the invention.